DESCRIPTION: Fragile X syndrome is the most common inherited cause of mental retardation. In addition to intellectual impairments which tend to become more apparent in later childhood, communication deficits are common although variable among males with fragile X syndrome (FXS). Currently, very little is known about the speech and language development of young males with FXS and the impact of genetic variation on their language development. The proposed research will examine longitudinally the communication development of preschool and elementary age males with FXS, focusing on language characteristics common in older males with FXS (e.g., perseveration of words and topics, declines in language growth), and examine how variations in genotype may explain these communication differences. This research also will compare the language development of males with FXS to males with Down syndrome (DS) and to typically developing matched controls. It is anticipated that males with FXS and DS will show overall communication delays and different profiles of communication development compared with typically developing males. In these communication profiles, nonverbal cognitive skills will be a strength for males with FXS and DS, followed by speech in isolated words and vocabulary, with syntax, particularly morphosyntax, being the weakest. However, the investigators expect that males with FXS and DS will differ in discourse skills; males with FXS will initiate interactions more frequently, maintain topics less often, and perseverate on words and topics more often. In addition, they hypothesize that FMR1 protein (FMRP) will contribute to the rate of change and profiles of communication skills of young males with FXS. Sixty preschool and elementary school age males with FXS, 40 males with DS, and 40 typically developing males matched for nonverbal intelligence age will be followed for five years. Vocabulary size and diversity, utterance length and complexity, topic maintenance and perseveration, speech accuracy and intelligibility, and overall receptive and expressive language will be compared for the three groups. Fragile X DNA testing and FMRP analysis from blood samples will be done only on males with FXS to determine the methylation status of the FMR1 gene and the percentage of lymphocytes producing FMRP. Growth curve methods will be used to quantify patterns of change over time in the overall level and rate of communication development, to develop profiles of cognitive, language, and speech development for each group, to determine the extent to which profiles differ among the groups over time, and to determine for males with FXS if variation in communication development can be accounted for by their genotype. Findings will contribute to a better understanding of the patterns of communication development in young males with FXS and provide an essential knowledge base for early communication intervention.